Local area networks (LAN) have made a major impact on office environments in the last 20 years. Generally, two types of LAN have been offered commercially, the most common being Ethernet™ and the second most common token rings. Most LAN's have network elements coupled by wires or optical cables but there are LAN designs which use infra-red or radio interconnections (so-called Wireless LAN's).
LAN technology has reached a high level of sophistication, e.g. server-based LAN's, interconnections between LAN's to provide a wide-area service (WAN) and virtual LAN's which support different protocols, e.g. both Ethernet™ and Token Rings.
Up to now a LAN has been an additional component in any office, that is the LAN has been installed alongside existing telephone and power services, and has therefore been an additional expense. There is an on-going interest in making the office environment more efficient, e.g. by combining services together thus reducing infrastructure costs. Attempts of this kind include providing telephone services via a LAN, e.g. Voice-Over-IP or VoIP which provides telephone services at a LAN terminal via data networks and gateways to telephone networks, or attempts have been made to provide telephone and data services via power circuits.
A more fundamental simplification of the office environment is to do away with the traditional wire-line telephone system in the office and replace it by a cordless or wireless system (the so-called “Cordless Office” or “Wireless Office”). In the article “The wireless office” by Gordon J. Povey in Electronics & Communication Eng. Journal, vol. 8, no. 2, April 1996, the concept is described of providing cellular phones to employees for general use within an office building rather than the employees having conventional wire-line telephones. Low power base stations are used in the office and the cellular phones are said to be compatible with the main cellular network outside the building. This suggestion has been criticized in the article by W. H. W. Tuttlebee entitled “Cordless telephones and cellular radio: synergies of DECT and GSM”, Electronics & Communication Eng. Journal, vol. 8, no. 5, October 1996, as being too expensive. It should be noted that providing wiring and installing all the equipment amounts to about 80% of the costs of a conventional indoor mobile phone network.
A system for providing base stations in an office environment is described in EP 766 427 in which a gateway between a mobile telecommunications system and a LAN is provided. Low-power base-stations are distributed around the LAN, i.e. within the office. the base stations and the gateway are designed to provide a radio interface between the low-power base stations and mobile terminals which is substantially identical with that provided by an “ordinary” base station in mobile telecommunications systems. By ordinary it is presumably meant that the radio interface is identical to that provided by base stations normally linked on the wire-line side to a hierarchical telecommunication system with dedicated communication links to the base stations.
This known system has certain disadvantages. In a GSM system, the LAN of the known system provides the GSM Abis interface, i.e. the interface between the BSC and the base station transceiver (BTS) and the wireline connection between the LAN gateway and the network MSC provides the A interface conventionally located between the mobile switching center (MSC) and the base station controller (BSC). For more explanation of the GSM system see, for example, the book by M. Mouly and M-B Pautet, “The GSM system for mobile communications”, Cell & Cys, 1992. The gateway and base stations of '727 form a Base Station Sub-system (BSS) in which each base station controls a small coverage area or nanocell. The whole LAN is a location area within the GSM network, i.e. it corresponds to a significant element of the network despite only serving a limited number of mobiles. This means that the distributed base stations have at least full base station functionality. In fact they have more: they have dynamic channel allocation and a LAN interface. This increases the cost of the base station and places a load on frequency reuse and on the number of location area indicators in the network. The result is an expensive system which may have difficulty competing with alternative systems, e.g. a cordless telephone system despite the fact that such a system does not provide the same functionality, e.g. dual mode mobile stations would be required to provide access to both the cordless and wireless systems. The operation of a plurality of nanocells increases the difficulty of finding suitable frequencies for re-use throughout the system.
GB 2 308 041 describes a central control unit which communicates with radio head over a dedicated HDSL link. A synchronization pulse is sent to each radio head every hour by the control unit over the HDSL link. Accordingly, timing signals must be generated local to the radio head for its operation which are very accurate over time periods of one hour. This requires an accurate timing device in each radio head. There is no indication that such a control unit (e.g. providing synchronization signals or channel coding or decoding) would work effectively when the radio heads are used as a shared resource on a Local Area Network.
GB 2 320 647 describes a cellular radio communication system having a central cell control station which takes over some of the functions of a base station transceiver. The central control station has a Local Area Network which is connected via a router with a remote base station over a dedicated ISDN/ATM link. The base stations are distributed over a wide area which requires generation of timing signals for the base station locally to the base station rather than in the central control station due to the large and unpredictable delays over the long distances.
The use of a LAN in mobile networks is known from U.S. Pat. No. 5,475,689 which describes the use of a LAN between the mobile switching center (MSC) and the base station controller (BSC). However, in this system the base stations are conventional.
U.S. Pat. No. 5,187,806 describes apparatus for extending a cellular system. This system employs a dedicated (antenna to antenna) radio frequency connection between a master base station and a remote base station. Another system described in U.S. Pat. No. 5,809,422 uses a line-of-sight microwave or optical link, an optical or a coaxial cable. A further system is known from U.S. Pat. No. 5,657,374 which uses an optical cable (or other non-specified high band-width carrier) between centralized base stations and distributed antenna units.
Provision of mobile communications within a building may be looked upon as one case of provision of such services in inaccessible places. U.S. Pat. No. 5,603,080 and U.S. Pat. No. 4,718,108 deal with systems able to provide mobile telephone communications in an inaccessible place, e.g. underground. Both known systems use a cable connection to repeaters in the inaccessible region.
It is an object of the present invention to provide a network and a method of operating the same which provides mobile telecommunications services in inaccessible places and/or buildings at a reasonable cost.